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Hydraulic Motors and Excavators

M Shade



by Michael Shade, IQS Editor

Have you ever watched an excavator at work and wondered what controls its motion? I’m imagining the half dozen excavators, front-loaders and backhoes that are parked in front of my apartment while my narrow street undergoes a major construction project. For the last several weeks, those machines have helped me build character in a number of ways. They’ve taught me patience as I’ve been forced to park long distances from my house, they’ve helped me wake up every morning (even on weekends) at 7:00 sharp and they’ve kicked up a perpetual dust storm that has endowed the landscape with a reddish film. Despite these annoyances, it’s fascinating watching them work. Watching an excavator’s hydraulic motors and cylinders force its components back and forth is kind of mesmerizing. Shortly after the first time I sat down on my porch to watch them, I decided to do some reading about what makes those big machines go.

Hydraulic Motor

An example of a large hydraulic motor (image source).

To put it simply, a lot of things make an excavator go. They’re equipped with a large assortment of hydraulic cylinders, cordage, electronics and actuators. When you look at the boom on an excavator, which is the long arm to which the excavator’s bucket is attached, it’s not too hard to understand what’s going on. When motion is actuated in the huge hydraulic cylinders, it causes the boom to move up and down. But the mechanism by which an excavator’s cab rotates is less obvious. This is where hydraulic motors come in.

A hydraulic motor is an apparatus that is used to actuate rotational motion. Its method of operation is not unlike that of hydraulic cylinders, which are used to actuate linear motion. Both kinds of equipment involve the use of pressurized hydraulic fluid, which is directed into the equipment through inlets. The force exerted by the pressurized fluid causes the moving parts of the equipment to move. A hydraulic motor is composed of an outer housing with two inlets and a rotor contained within the housing. When hydraulic fluid is forced into the housing through an inlet, it causes the rotor to turn, which in turn causes any attached equipment to rotate as well. Depending on the size and design of the motor, they can be used to generate fairly small amounts of torque, or they can be large enough to rotate the cab of an excavator.

In the case of excavators, the hydraulic motor is the point at which the cab makes contact with the track or wheel platform. If, for example, an excavator was being used to transfer dirt from a dig site into the back of a dump truck, the hydraulics in the boom would allow the bucket system to pick up the dirt, and the hydraulic motor between the platform and the cab would allow the boom to rotate to where the dump truck is located.

An excavator hydraulic motor is not an example of a high speed hydraulic motor. In fact, excavator motors rotate rather slowly. But they often boast an impressive combination of high power and precision. After all, they’re used to move tremendously heavy materials, and especially in the case of small excavators, they’re often deployed in situations where false moves could have consequences for nearby utilities like water mains and gas lines. For this reason, some excavator hydraulic motors are capable of generating cab rotation without wobbling, and their motion can be carefully and precisely controlled.

I won’t say that I enjoy being subject to the noise they make or the dust they generate, but I can’t help be fascinated by these kinds of heavy equipment and how they work. And there’s no denying that the sum of all of their parts – the servos, the cylinders and they hydraulic motors – are indispensable parts of modern life.